As is now well known to those skilled in the art, extruded and molded rubber compositions have been widely used in a variety of applications, particularly in the automotive field, for gaskets, seals, hoses, grommets, tubing, rub strips and bumpers. One type of rubber which has enjoyed considerable success in those applications due to its favorable processing characteristics and vulcanizate properties has been the so-called ethylene-propylene-diene monomer terpolymer rubbers or EPDM rubbers. Those rubbers are well known to those skilled in the art, and are formed by interpolymerization of ethylene, one or more mono-olefins containing 3-16 carbon atoms, and preferably propylene, and one or more polyenes containing a plurality of carbon-to-carbon double bonds
Preferred as the diene monomer in such EPDM rubbers are the open chain polyunsaturated hydrocarbon containing 4-20 carbon atoms such as 1,4-hexadiene. Even more preferred are the monocyclic and polycyclic polyenes, and preferably polyunsaturated bridged ring hydrocarbons or halogen substituted bridged ring hydrocarbons. Examples of the latter include the polyunsaturated derivatives of bicyclo-(2,2,1)-heptane wherein at least one double bond is present in one of the bridged rings, such as bicyclopentadiene, bicyclo-(2,2,1)-hepta-2,5-diene, the alkylidene norbornenes, and especially the 5-alkylidene-2-norbornenes wherein the alkylidene group contains 1-20 carbon atoms and preferably 1-8 carbon atoms, and the alkenyl norbornene, and especially the 5,alkenyl-2-norbornenes wherein the alkenyl group contains about 3-20 carbon atoms and preferably 3-10 carbon atoms. Other bridged ring hydrocarbons suitable for use as the diene monomer include polyunsaturated derivatives of bicyclo-(2,2,2)-octane such as bicyclo-(3,2,1)-octane, polyunsaturated derivatives of bicyclo-(3,3,1)-nonane and polyunsaturated derivatives of bicyclo-(3,2,2)-nonane.
Specific examples of preferred bridge ring compounds include 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 5-n-propylidene-2-norbornene, dicyclopentadiene and the methylbutanyl norbornenes such as 5-(2-methyl-2-butanyl)-2-norbornene, 5-(3-methyl-2-butanyl)-norbornene and 5-(3,5-dimethyl-4-hexanyl)-2-norbornene.
A number of such EPDM rubbers are commercially available from Copolymer Rubber and Chemical Corporation under the trademark "EPsyn.RTM.".
For the end uses cited above, fabrication is not complete until the rubber article has been vulcanized or cured to enhance mechanical strength and stability necessary for prolonged usage.
The side chain unsaturation of EPDM provides for curing by a variety of mechanisms including peroxide, sulfur, and resins. The choice of vulcanizing system is important since it affects stress-strain properties of the final vulcanizate as well as heat resistance and compression set. Resin crosslinking systems are employed for EPDM when improved heat resistance is required and when the EPDM is formulated in combination with other rubbers which cannot be crosslinked with sulfur or peroxide. Peroxide systems often are not employed with EPDM due to odor problems and requirements for special vulcanization techniques, for example, hot air cures cannot be utilized with peroxide systems. Formulations requiring high levels, greater than twenty percent of naphthenic or paraffinic processing oils and carbon black are also slow to cure even with high levels of peroxide. Sulfur crosslinking systems are used more broadly with EPDM since no special techniques or processing equipment is required for formulating, extruding or molding, and vulcanization. By adjusting the level of unsaturation in the base EPDM, sulfur systems can very economically and effectively be used to control the degree of cure in the fabricated article, without concern of crosslinking during extrusion or molding. By proper choice of the accelerator, very rapid vulcanization cycles can be achieved.
A typical vulcanization recipe for a sulfur cure system would include (1) an activator, commonly metal oxides such as zinc oxide, magnesium oxide, manganese oxide, and fatty acids such as stearic acid used in conjunction with the metal oxide if an organic accelerator is used, (2) sulfur or a sulfur masterbatch, and (3) an accelerator, needed in order to produce a specific degree of cure in a practical time for commercial use.
In the formulation of EPDM for applications of interest here, it is generally necessary to incorporate carbon black and plasticizers or processing oils. The carbon black is used as a reinforcing agent and to provide stability against detrimental radiation and ozone. Processing oil reduces the effective viscosity of the blend so that high Mooney viscosity, more economical and readily available types of EPDM rubber can be used.
In applications where a carbon black formulation is used, an iridescent sheen has been observed on dense and cellular extruded and dense molded parts both prior to and after vulcanization. The sheen is a surface phenomenon which exhibits visual colors of gold, greens and blues. The greater the surface area of the extruded or molded part the more intense the condition of iridescent sheen. Even though the sheen does not seem to affect the physical properties of the vulcanizate, its chromatic appearance has been found objectionable by the automotive industry. Broader use of EPDM in many automotive applications is hindered by the oil on water appearance associated with the iridescent sheen phenomenon. Color coding of the various automotive parts is of particular concern to today's automotive design engineers. Black parts would be specified more often if the quality of the black surface could be made compatible with the other colors.
The iridescent sheen phenomenon occurs particularly with exposure of the fabricated article to ultraviolet light (normal fluorescent light has a sufficient UV intensity to activate the sheen) and ozone. It has been determined that the appearance of the sheen can be accelerated by placing a sample of the molded or extruded part in an ozone chamber with an ozone concentration level of 50 pphm for four hours.
In studying the phenomena of iridescent sheen numerous phases of the fabrication process were examined including formulations, methods of compounding, conditions of extrusion or molding, and methods and conditions of curing. It had been recognized previously that components of the formulation and more typically the sulfur and plasticizers would bloom; migrate to the surface of a molded or extruded part. Much of the compounding literature teaches ways in which sulfur bloom can be minimized by proper choice of sulfur, for example, reduced use of sulfur by incorporation of organic sulfur vulcanizing agents. The iridescent sheen observed here is a problem distinguished from that of sulfur bloom and usually both are not observed with the same formulation. However, working with a hypothesis that the sheen was a result of some component or components bleeding to the surface of the rubber part, laboratory investigators have revealed that the sheen phenomenon could be washed out or extracted with certain solvent. The extraction solvents were then analyzed to contain components of the accelerators and plasticizer systems.
It is accordingly an object of the present invention to provide a rubber compounding composition which overcomes the foregoing disadvantages under conditions commonly practiced in manufacture, storage and end use of articles.
It is a more specific object of the present invention to provide a crosslinking system where levels of the components can be increased or decreased without developing the iridescent sheen thereby permitting the user to modify the rate of cure to fit the processing needs during forming and vulcanization.
It is a more specific object of the present invention to provide a rubber compounding and rubber curing composition which avoids the formation of iridescent sheen.
It is yet another object of the present invention to provide a rubber compounding composition containing a unique combination of accelerators to minimize the formation of iridescent sheen.
These and other objects and advantages of the invention will become more apparent hereinafter.